Heretofore, as shown in U.S. Pat. No. 3,943,974 dated Mar. 16, 1976, entitled "Control Valve for Fluid Actuator" and U.S. Pat. No. 3,963,050 dated June 15, 1976, entitled "Pneumatic Relay", control valves have been provided for bleeding fluid from a valve actuator to permit a controlled valve to move to a closed position across the main flowline. Such prior control valves have included a slide valve having a piston on one end thereof mounted within a piston chamber for longitudinal movement between a seated position adjacent an outer end of the piston chamber in which fluid is exhausted from the actuator, and a raised unseated position adjacent the inner end of the piston chamber to permit fluid to be supplied to the actuator for moving and holding the controlled valve to an open position across the main flowline.
In the down or seated position in such prior art arrangements, the outer face of the piston is exposed to fluid pressure from a port leading to the outer end of the piston chamber and a bleed vent communicates with the inner end of the piston chamber adjacent the inner face of the piston. However, the piston chamber shown in such control valves heretofore has been of a dual diameter with a small diameter section forming the inner end of the piston chamber and a large diameter section forming the outer end of the piston chamber. The piston in seated position has been spaced from the inner wall defining the large diameter section of the piston chamber to permit fluid from the port to by-pass the piston and be vented to atmosphere through the bleed vent. By this arrangement, any fluid pressure leaking into the piston chamber from the port in the seated position of the piston results in fluid pressure being vented around the piston to allow the piston to remain in its seated position. The piston is positioned within the small diameter section of the piston chamber in its raised unseated position and is in fluid-tight sealing engagement with the inner wall defining the small diameter section.